A metal paste containing metal (a single metal or alloy) as a mixed dispersion is manufactured by kneading and dispersing a paste-like material (referred to as a paste material hereafter) containing a metal filler, as a material of a dispersion. As a kneading/dispersion method, a method using a ball mill, a method using a kneader, a method of using a roll mill recited in patent document 1, and a method using a colloid mill recited in patent document 2 are used according to the purpose of use.
Patent document 1 Japanese Patent Laid Open No.11-277527
Patent document 2 Japanese Patent Laid Open No.2000-153167
However, when the metal paste is manufactured by kneading and dispersing the paste material by means of the ball mill, in some cases, entry of impurities occurs into the metal paste by a friction caused by collision of balls themselves or by collision of balls and a wall surface. In addition, a problem arises such that when a particle diameter of the metal filler is made to be small in a target dispersion state of the metal paste, the balls must also be made to be small accordingly, thus deteriorating productivity with smaller balls.
When the metal paste Is manufactured by using the kneader, an undispersed stagnation place is generated on the wall surface of a vessel. Thus, it becomes difficult to uniformly disperse the metal filler over the entire body of the paste material.
When the metal paste is manufactured by using the roll mill, the gap between rolls is adjusted to reduce the particle diameter of the metal filler to a specified size. Thus, the metal paste is passed through the rolls for a plurality of times while gradually reducing the gap, thus deteriorating the productivity.
Here, when the gap between the rolls is reduced once to improve the productivity, since the metal filler exhibits spreading property, it is collapsed to generate a foil of a filler (metal foil), and a product becomes defective.
In the metal paste having an excellent printability, which is generally required, viscosity characteristics thereof is required to be 60 (Pa·s) or larger at a measured value by a rotational viscometer rotating at a shear rate of 2 s−1, or 30 (Pa·s) or larger at the measured value by the rotational viscometer rotating at a shear rate of 10 s−1. Also, when the measured value by the rotational viscometer rotating at a shear rate of 10 s−1 is set to be η2, and when the measured value by the rotational viscometer rotating at a shear rate of 10 s−3 is set to be η10, the “thixotropic ratio” defined as the value of (η2/η10) is required to be in a range from 1.5 to 2.5.
Further, in the metal paste having the excellent printability, a maximum particle diameter of the metal filler contained therein is required to be 50 μm or less.
However, even if the gap of whetstones in the colloid mill is adjusted and the paste material is fed in the gap between colloid mills in order to manufacture such a metal paste having excellent printability by the colloid mill, it is difficult to pass the paste material therethrough due to high viscosity of the paste material. Thus, the metal paste can not be manufactured.
Here, as a precursor to the present invention, the inventors of the present invention try to manufacture the metal paste, by adjusting an amount of a solvent to be added such as alcohols contained in the paste material, decreasing the viscosity of the paste material, and thereafter using the colloid mill. As a result, it is found that when the value is 40 (Pa·s) or less at the shear rate of 2 s−1, and the value is 20 (Pa·s) or less at the shear rate of 10 s−1, it becomes possible to manufacture the metal paste by using the colloid mill. However, in the metal paste manufactured from the paste material satisfying a viscosity range as described above, the content of the solvent is excessive, and this is undesirable in terms of its original purpose of use.